A nitrate waste liquid generated from nuclear facilities such as a reprocessing plant has high concentration (salt concentration of equal to or more than 1%) as well as being radioactive. Therefore, the nitrate waste liquid cannot be discharged as it is, and is finally cast into a cement-solidified form and disposed underground.
In recent years, there has been a concern that when the cement-solidified form contains nitrate, the nitrate may leak and pollute surrounding ground water and soil environment and thus dissolution of the nitrate is studied.
There are various methods for treating nitrate such as an electric reduction method, a chemical reduction method, and a biological reduction method. The electric reduction method has problems such as inhibition by heavy metals and generation of ammonia.
Besides, the chemical reduction method has problems such as exothermic reaction and generation of ammonia.
On the other hand, the biological reduction method allows treatment at an ordinary temperature and pressure and there is no generation of ammonia. Accordingly, the method of treating nitrate by using the biological treatment has been studied (Patent Document 1). However, in the method of Patent Document 1, only one type of carbon source is used, so the method has problems such that the amount of redundant sludge generated with the nitrate reduction treatment is increased and a secondary waste disposal expense is increased.
FIG. 7 is a schematic diagram of a configuration of a conventional apparatus for treating a nitrate waste liquid utilizing an organism.
As shown in FIG. 7, a conventional apparatus 100 for treating a nitrate waste liquid includes a denitrification tank 102 that reduces nitric acid, which is present in a nitrate waste liquid 101 generated from nuclear facilities (not shown), to nitrogen gas, a reaeration tank 104 in which a denitrified liquid 103 obtained by denitrification is aerated and mixed with active sludge, and a precipitation tank 108 in which a reaerated liquid 105 discharged from the reaeration tank 104 is separated into precipitated sludge 106 and a treated liquid 107.
The denitrification tank 102 includes active sludge containing a large amount of denitrifying bacteria (not shown). In the denitrification tank 102, nitrate ions in a nitric-acid waste liquid are reduced to nitrogen gas (N2) according to a reaction based on the following formula (1) by an action of an anaerobic microorganism (denitrifying bacteria) and is reduced from the nitric-acid waste liquid.
At this time, a carbon source 121 such as methanol and a pH adjuster 120 are supplied to the denitrification tank 102. Further, a mixer 110 is used to mix inside the denitrification tank 102.NO3−+⅚CH3OH→½N2+⅚CO2+7/6H2O+OH−  (1)
Thereafter, the reaerated liquid 105 passes through the precipitation tank 108 from the reaeration tank 104, and is sent as a treated liquid 107 to a subsequent process (not shown). The sludge 106 precipitated in the precipitation tank 108 is recovered as redundant sludge 131 by a circulating pump 111. Further, a part of the sludge 106 precipitated in the precipitation tank 108 is returned to the denitrification tank 102 via a returned-sludge supplying line 112 and is reused. The sludge not reused is removed from a system as the redundant sludge 131 and is sent to a disposing process (not shown).
Further, there is used a method and an apparatus for treating a nitrate-containing waste liquid. The method includes a microbial treatment process of reducing nitrate nitrogen and nitrite nitrogen to nitrogen by using anaerobic denitrifying bacteria in microorganism-containing sludge, and the apparatus includes a plurality of treatment tanks (Patent Document 2).
Patent Document 1: Japanese Patent No. 3697037
Patent Document 2: Japanese Patent Application Laid-open No. 2007-105627